Preventing cheating in online chess games?

I can't see that there is anyway to prevent someone to using a chess engine to assist them, unless you can observe the player.

You might have some luck protecting against fully automated bots, though.

Sites like chesscube monitors you for some time if you comes under the radar of suspicion. They monitor how much time you are taking for hard moves and relative simple moves. If there isn't some serious difference, they may conclude you are cheating. Also I believe they implement some method to check the shifting between windows, however I'm not sure about what they use for it. But I personally know guys who had been banned. So their method is pretty good.

I second what JesperE say, You have to monitor the guy for sometime before arriving on an opinion.

Many chess computers work to formulas and end game books, so they will often play the same move in a particular situation. You could run users game history through a variety of chess computers and see if the users chosen moves after the opening moves have correlation with how the various chess computers play. This could be used to highlight users that are using chess computers.

Online poker sites use anti-bot measures similar to what you're describing. I recommend the series of articles How I Built a Working Poker Bot for a good overview of how these systems work, and how they are defeated.

I agree with the others who said that there's not much you can do to stop the most dedicated cheaters, but you might be able to prevent casual cheating. (The problem with that, of course, is that then the dedicated cheaters will rule your site.)